The Most Stable Coordination Complexes P and Q

Question Overview

The reaction involves the formation of coordination complexes from:

Fe³⁺ + Hg²⁺ + SCN⁻ → P + Q

Thiocyanate ion (SCN⁻) is an ambidentate ligand, meaning it can coordinate through either:

• Sulfur (–S) → written as SCN
• Nitrogen (–N) → written as NCS

We must identify the most stable complexes formed with Fe³⁺ and Hg²⁺.

Key Concept Involved

Ambidentate Ligands & HSAB Theory

SCN⁻ Coordination Modes

• Through Nitrogen (NCS) → preferred by hard acids
• Through Sulfur (SCN) → preferred by soft acids

Nature of Metal Ions

Stability Rule (HSAB Principle)

Hard acids bind strongly to hard bases, and soft acids bind strongly to soft bases

Therefore:

• Fe³⁺ prefers N-bonded SCN⁻ (NCS)
• Hg²⁺ prefers S-bonded SCN⁻ (SCN)

✅ Correct Answer: Option (C)

Why Option (C) is Correct

• Fe³⁺ complex shows coordination via NCS (N-bonded)
• Hg²⁺ complex shows coordination via SCN (S-bonded)
• Both metal–ligand interactions follow HSAB theory
• This combination gives maximum thermodynamic stability
• ✔ Hence, Option (C) represents the most stable coordination complexes P and Q

Explanation of All Options

Option (A) – Incorrect ❌

• Fe³⁺ coordinated via SCN (S-bonded)
• Violates HSAB principle
• Hard acid–soft base interaction → less stable

Option (B) – Incorrect ❌

• Both metals partially coordinated through incorrect donor atoms
• Mixed and unfavorable bonding
• Not the most stable configuration

Option (C) – Correct ✅

• Fe³⁺–NCS (hard–hard interaction)
• Hg²⁺–SCN (soft–soft interaction)
• Maximum stability
• Perfectly follows HSAB theory

Option (D) – Incorrect ❌

• Fe³⁺ bonded through sulfur
• Hg²⁺ bonded through nitrogen
• Completely opposite of preferred coordination
• Least stable arrangement

Final Conclusion

Using HSAB theory and the ambidentate nature of SCN⁻, the most stable complexes formed are:

• Fe³⁺ coordinated via NCS
• Hg²⁺ coordinated via SCN

🏆 Correct Answer: Option (C)